Multiple Sclerosis Multiple sclerosis (MS) is characterized by a chronic inflammatory demyelination, that damages the central nervous system (CNS) (Noseworthy et al., 2000). It is the most common cause of disability in young adults. It is one of the most debilitating medical conditions, not only physically, but also in terms of psychosocial implications.
MS is the most common neurological disease in young adult and, as shown by the descriptive studies, the geographical distribution of the disease is heterogeneous (Rosati, 2001). It concerns mainly the countries in central and northern part of Europe and those non-European regions that in various historical periods have been subject to a significant settlement of populations of northern European ancestry. MS is typically a disease of temperate climates; in both hemispheres its prevalence decreases with decreasing latitude. The comparison between the populations of North America and Europe indicate similar rates of prevalence and similar north-south gradient. Some areas of the world represent real focus of the disease, suggesting that environmental factors might be involved in MS.
In Caucasians, the average rates of total prevalence vary between 30 and 180 cases/100.000 inhabitants (Rosati, 2001) and the incidence is 10-20 new cases/100.000 inhabitants per year. It is most frequently diagnosed between 20 and 40 years, rarely affects children and the elderly. MS is about twice as common in women than in men (Pugliatti et al. 2006).
MS can be considered the result of a complex multifactorial interactions between genetic and environmental factors. The findings of several studies seem to demonstrate that MS is an immune-mediated disease related to T lymphocytes action and induced by external and unknown agents, such as viruses and bacteria, in genetically susceptible individual.
Despite the number of studies on the disease and a multidisciplinary approach to the problem, some pathogenic mechanisms of multiple sclerosis are still obscure and the aetiology is unknown.
At the present time, there are no definitive diagnostic tests for multiple sclerosis. Therefore, it is necessary to use different diagnostic tools: clinical (Trojano and Paolicelli, 2001), laboratory (Luque and Jaffe, 2007) and instrumental diagnosis (Achten and Deblaere, 2008).
1) Clinical diagnosis allows to evaluate: Patient medical history; Evidence of altered sensibility; impaired strength and vision disturbances; Symptoms/signs attributable to white matter lesions are not justified by other diseases; Spatial dissemination of lesions with clinical signs referable to 2 or more lesions; Symptoms/signs attributable to the temporal dissemination of the lesions: two or more relapses.
2) Laboratory diagnosis based on CSF investigations (inflammatory and autoimmune disorders) analyze the intrathecal synthesis of Immunoglobulin G (IgG) and the presence of oligoclonal bands.
3) Instrumental diagnosis comprise: Magnetic Resonance Imaging (MRI) that allows to show pathological foci in the brain stem, cerebellum and spinal cord and the presence of lesions in the corpus callosum and around the ventricles. In addition, through the use of contrast medium it is possible to highlight local impairments in Blood Brain Barrier (BBB) that precede signs of exacerbation and possible injury to the optic nerve; Computerized Axial Tomography (CAT) shows less dense areas around the ventricles corresponding to the plaques in which the myelin is no longer present; Testing of Evoked Potentials (EP) measures the transmission time of sensory messages that travel through the nerves.
The current MS diagnostic procedure is rather long and tortuous thus there is the need for an early diagnosis of MS since an earliest possible therapeutic intervention would be most effective for the long term, even with currently available therapies.
The currently available therapy (β-interferon, steroids, symptomatic therapy) act on the symptomatology and are aimed at slowing the progression of the disease thus, in view of the above considerations, also a etiological therapy would be needed in order to treat permanently the disease.
5HT2a Receptor and Multiple Sclerosis
5HT2A receptor (5HT2aR) belongs to the family of serotonin receptors. There are at least 13 different receptors for serotonin grouped into 7 families based on the mechanism of signal transduction. Except 5-HT3, which is a ligand-gated ion channel, the other members are all G protein-coupled receptors, and in particular, the 5HT2aR is coupled to the Gq/11 (Barnes and Sharp, 1999).
The 5HT2aR activates multiple transduction pathway: a) the PLA2 pathway leading to arachidonic acid (AA) production; b) the PLC pathway, that through an action on phosphatidylinositol 4,5-bisphosphate (PIP2), generates diacyl glycerol (DAG) and increases intracellular Ca2+ levels activating protein kinase C (PKC); c) it also activates membrane calcium channels promoting calcium influx (Raote et al., 2007). 5HT2aR is expressed by oligodendrocytes where its downstream signaling exerts modulatory effects on myelin formation (Millan et al., 2008).
Further, in relapsing-remitting (RR) MS patients, there is a dysmetabolism of the serotonergic pathway. In these patients disability accumulation during disease progression correlates negatively with the CSF levels of 5-hydroxyindoleacetic acid (5-HIAA), an index of serotonergic activity in the CNS (Markianos et al., 2008).
5HT2aR is used by the JC virus (JCV, from the initials of the patient—John Cunningham—from which the virus was for the first time isolated) polyoma virus to infect cells. JCV has a very restricted tropism, since it is able to replicate only within glial cells (although it can also infect other cell types such as B lymphocytes, hematopoietic progenitor cells and few others) and, in addition, it is the “causal agent” of progressive multifocal leukoencephalopathy (PML), an often fatal disease associated with oligodendrocyte lysis and widespread demyelination (Elphick et al., 2004). The link between MS and JCV has emerged following the onset of PML in MS patients treated with NATALIZUMAB, a humanized monoclonal antibody used in the treatment of autoimmune inflammatory disorders such as MS, Parkinson's and Crohn diseases (Achiron et al., 2005).
Natalizumab is an antibody directed against the integrin alpha4 beta1 also known as VLA4 and thus works by preventing the adhesion and migration of lymphocytes from the vascular bed to the site of inflammation. By blocking the migration of T cells, the central nervous system remains partially immunologically unprotected promoting replication and viral reactivation. The infection of glial cells with JCV depends on the binding of the virus to a receptor complex comprising a carbohydrate receptor and the 5HT2aR and is blocked by antagonists of the 5HT2A receptor as well as by blocking clathrin-dependent receptor-mediated endocytosis.
JCV is present in 80% of population, and the infection is subclinical, but most studies aimed at finding the virus in the CSF have shown that it is never present in the CSF of normal subjects while it is present, although in a reduced number of cases, in subjects with MS (Alvarez-Lafuente et al., 2007).
It is noteworthy that the average viral load is very low (4-6 copies/ml) and close to the limit of sensitivity of the PCR technique. Therefore, the positivity for JCV in MS patients is most likely underestimated.
NOXs in Oligodendrocytes
Oxidative stress is implicated in many neurological diseases, including Multiple Sclerosis. Together with mitochondria, NOX enzyme play a role in reactive oxygen species (ROS) production in the CNS.
NOX enzymes are membrane NADPH oxidases producing superoxide anions by one electron reduction of oxygen using NAD(P)H as the electron donor (Bedard and Krause, 2007). Regulated production of reactive oxygen species (ROS) by NADPH oxidase was first discovered in phagocytic cells. Phagocytic NADPH oxidase is a multicomponent complex comprising two integral membrane proteins, the catalytic subunit gp91phox (CY24B_HUMAN Cytochrome b-245 heavy chain, now referred to as NOX2) and p22phox, and the cytosolic components p47phox, p67phox, p40phox and the small GTPases Rac1 or 2. Upon stimulation, cytosolic subunits translocate to the membrane, activating the enzyme (Babior et al., 2002).
More recently, other isoforms of the catalytic subunit, other than NOX2, have been discovered and up to now in mammalian, seven different NOX genes (NOX1 to 5 and DUOX1 and 2) have been identified (Lambeth, 2004). Like NOX2, also NOX1, NOX3 and NOX4 are associated with the membrane subunit p22phox, but the mechanisms of activation are different. NOX1 is activated by membrane translocation of the cytosolic subunits NOXO1, NOXA1 and Rac 1 or 2, while NOX3 requires NOXO1 but the role of the other cytosolic subunits is still uncertain. NOX4, NOX5, DUOX 1 and DUOX 2 activity is not modulated by cytosolic subunits (Bedard and Krause, 2007). NOX4 is constitutively active. NOX5, DUOX1 and DUOX2 are modulated by calcium that interacts with EF-hand binding domains (a helix-loop-helix structural motif found in a large family of calcium-binding proteins) at the N-terminus of the proteins.
Many membrane receptors relay on NOX-dependent ROS production for downstream signaling. As examples, NOX enzymes are activated by growth factor receptors such as platelet-derived growth factor receptor (Svegliati et al., 2005, Baroni et al., 2006; Gabrielli et al.; 2008, Damiano et al.; 2012), epidermal growth factor receptor (Damiano et al., 2015), cholinergic receptors (Serù et al., 2004) and many others (Petry et al., 2010). Also 5HT activates downstream signaling through NOX2—produced ROS (Regmi et al., 2014; Fang et al., 2013; Kruk et al., 2013). NOX enzymes are widely expressed in central nervous system cells, including oligodendrocytes (Sorce and Krause, 2009); in particular, NOX2 is involved in NMDA receptors signal transduction in these cells (Cavaliere et al., 2013).
In current practice, diagnosing, managing and treatment of Multiple Sclerosis is challenging, there are no definitive tests, symptoms vary widely across individuals and within patients over time, and measurement of disease progression is problematic. Differential diagnosis often entails extensive clinical observation and a battery of costly tests like MRIs. Poor insight into disease progression and therapeutic response creates uncertainty in designing and implementing therapeutic strategies.
Based on the above there is still the need for improved diagnostic and therapies for MS.